I. Field of the Invention
The present invention relates to an interframe coding method and an apparatus therefor, wherein highly precise prediction is performed for moving areas of a picture to optimally code a video signal thereof with high efficiency.
II. Background of the Invention
Interframe coding is proposed as one of conventional coding schemes to code a video signal with high efficiency by using interframe correlation. In interframe coding, a pel value of a previous frame to the present frame is used as a predictor of an input video signal. This coding scheme is based on an assumption that a displacement in a picture is small. For example, interframe coding is mainly used for television conferences wherein a displacement of moving objects is small. For this reason, when an object is greatly displaced, proper prediction cannot be performed, thereby degrading the coding efficiency.
A motion compensated interframe coding method is proposed in Japanese Patent Disclosure No. 56-93492 so as to improve prediction precision even when an object is greatly displaced. According to this method, an input signal of a given frame is divided into blocks each of which consists of 7 lines.times.7 pels. For each block of the given frame, a block of the previous frame which is located in the same position as the block of the given frame, and blocks displaced by .+-.m vertical lines (e.g., m=1 to 6) and .+-.n horizontal pels (n=1 to 6) from this block of the previous frame are set. An optimal block which minimizes a prediction error is selected from these blocks of the previous frame. Vector data representing vertical and horizontal displacements between the block of the current frame and the selected optimal block of the previous frame is supplied to a receiving end. Predictive image coding of the current frame is performed using as a predictive value the value of pels in the selected block of the previous frame.
However, according to the conventional method of this type, when a member participating in a conference is in motion, motion compensation is performed for this moving person, but cannot be applied to a background area such as a wall monitored on the screen. Therefore, a large amount of data is generated from the background area and thereby degrading coding efficiency and resulting in inconvenience.